How can I do a detailed landscape?
As of 2.78a, you can use adaptive subdivision by turning on experimental features, using a subsurf modifier, and checking the "adaptive" checkbox in the subsurf options. Then, you can head up to the object's node tree and connect a procedural texture to the "displacement" input of the material output. Depending on what kind of landscape you want to do, you can mix procedural textures using the mixrgb node. Noise alone yields a connemara-ish landscape, Voronoi + waves creates a desert and Voronoi + Noise creates alpine mountains.